1. Field of the Invention
This invention relates generally to optical transport networks and more particularly to a new architecture for optical transport networks that deploys low cost, highly compact optical-electrical-optical (OEO) regeneration (REGEN) modules and terminals compared to those presently known or deployed today.
2. Description of the Related Art
If used throughout this description and the drawings, the following short terms have the following meanings unless otherwise stated:
1R—Re-amplification of the information signal.
2R—Optical signal regeneration that includes signal reshaping as well as signal regeneration or re-amplification.
3R—Optical signal regeneration that includes signal retiming as well as signal reshaping as well as regeneration or re-amplification.
4R—Any electronic reconditioning to correct for transmission impairments other than 3R processing, such as, but not limited to, FEC encoding, decoding and re-encoding.
A/D—Add/Drop.
APD—Avalanche Photodiode.
AWG—Arrayed Waveguide Grating.
BER—Bit Error Rate.
CD—Chromatic Dispersion.
CDWM—Cascaded Dielectric Wavelength Multiplexer (Demultiplexer).
CWDM—Coarse Wavelength Division Multiplexing—transmission of data on more than one wavelength in a given direction on a grid of spacing greater than 200 GHz.
DBR—Distributed Bragg Reflector laser.
EDFAs—Erbium Doped Fiber Amplifiers.
DAWN—Digitally Amplified Wavelength Network.
DCE—Dispersion Compensating Elements either for CD, such as DCFs, dispersion managed solitons, frequency guiding filtering, chirped fiber Bragg gratings, or dispersion slope compensation, or for PMD, such as through optical device design to achieve polarization insensitivity or equalization optical circuitry to provide distorted optical signal polarization transformation.
DCF—Dispersion Compensating Fiber.
DEMUX—Demultiplexer.
DFB—Distributed Feedback laser.
DCF—Dispersion Compensating Fiber.
Digital OEO REGEN—an OEO REGEN that provides complete digital handling of channel signals including their regeneration in a digital optical network without deployment or need for analog optical components including optical fiber amplifiers.
DM—Direct Modulation.
DON—Digital Optical Network as defined and disclosed in this application.
DWDM—Dense Wavelength Division Multiplexing—transmission of data on more than one wavelength in a given direction on a grid of spacing less than or equal to 200 GHz.
EDFA—Erbium Doped Fiber Amplifier
EML—Electro-absorption Modulator/Laser.
EO—Electrical to Optical signal conversion (from the electrical domain into the optical domain).
FEC—Forward Error Correction.
GVD—Group Velocity Dispersion comprising CD and/or PMD.
ITU—International Telecommunication Union.
MMI—Multimode Interference combiner.
Modulated Sources—EMLs or SMLs, combinations of lasers and external modulators or DM lasers.
LR—Long Reach.
MZM—Mach-Zehnder Modulator.
MUX—Multiplexer.
NE—Network Element.
NF—Noise Figure: The ratio of output OSNR to input OSNR.
OADM—Optical Add Drop Multiplexer.
OE—Optical to Electrical signal conversion (from the optical domain into the electrical domain).
OEO—Optical to Electrical to Optical signal conversion (from the optical domain into the electrical domain with electrical signal regeneration and then converted back into optical domain) and also sometimes referred to as SONET regenerators.
OEO REGEN—OEO signal REGEN is OEO conversion wherein the signal is regenerated in the electronic domain between conversions to the optical domain. SONET regenerators are one example of OEO REGEN but it is not limited to such regenerators.
OO—Optical-Optical for signal re-amplification due to attenuation. EDFAs do this in current WDM systems.
OOO—Optical to Optical to Optical signal conversion (receiving in the optical domain, processing in the optical domain, and transmitting in the optical domain, without conversion of the signal to the electrical domain).
OOO-REGEN—OOO signal REGEN using all-optical regeneration.
OSNR—Optical Signal to Noise Ratio.
PIC—Photonic Integrated Circuit.
PIN—p-i-n semiconductor photodiode.
PMD—Polarization Mode Dispersion.
REGEN—optical signal regeneration or regenerator is signal restoration, accomplished electronically or optically or a combination of both, which is required due to both optical signal degradation or distortion primarily occurring during optical signal propagation caused by the nature and quality of the signal itself or due to optical impairments incurred on the optical transport medium.
Rx—Receiver, here in reference to optical channel receivers.
RxPIC—Receiver Photonic Integrated Circuit.
SDH—Synchronous Digital Hierarchy.
SDM—Space Division Multiplexing.
Signal regeneration (regenerating)—Also, may be referred to as signal rejuvenation. This may entail 1R, 2R, 3R or 4R and in a broader sense signal A/D multiplexing, switching, routing, grooming, wavelength conversion as discussed, for example, in the book entitled, “Optical Networks” by Rajiv Ramaswami and Kumar N. Sivarajan, Second Edition, Morgan Kaufmann Publishers, 2002.
SOA—Semiconductor Optical Amplifier.
SONET—Synchronous Optical Network.
SR—Short Reach.
TDM—Time Division Multiplexing.
Tx—Transmitter, here in reference to optical channel transmitters.
TxPIC—Transmitter Photonic Integrated Circuit.
VOA—Variable Optical Attenuator.
VMPD—Velocity-Matched Distributed Photodetector.
WDM—Wavelength Division Multiplexing—transmission of data on more than one wavelength in a given direction.
The landscape of optical transport networks has changed significantly over the past ten years. Prior to that time, most long haul telecommunications were generally handled via electrical domain transmission, such as provided through wire cables, which are bandwidth limited. Telecommunication service providers since about 1990 have vastly higher information or data transmission capability particularly in bandwidth compared to traditional electrical/electronic transport networks. Capacity demands now have increased significantly with the advent of the Internet which has been said to bring about six times, and probably more, as much traffic into a network compared to electrical domain transmission. The demand for information signal capacity increases dramatically every year.
Optical transport networks deployed prior to 1994 were time division multiplexing (TDM) or space division multiplexing (SDM) where optical data signal at a single wavelength or at a different wavelength were each transported in a single fiber in opposite directions such as 1300 nm in one direction and 1500 nm in the other direction. This type of network 10 is shown in FIG. 1. In FIG. 1, the information signal, for example, to be transmitted in the optical domain is received in terminal 12 electronically via linecard 14 and converted to the optical domain for transmission on an optical fiber link, via an optical module 12A by either direct modulation of a discrete semiconductor laser, such as a DFB laser, or by external modulation using an optical modulator, such as a discrete Mach-Zehnder modulator (MZM) receiving light from a discrete, CW operated DFB or DBR laser. Optical module 12 also may include an optical receiver for channel signals propagated in an opposite direction from terminal 16. These discrete optical components are optically aligned at the factory and provided in a package 12A and mounted in association with linecard 14 as a module on a mother board, for example, for insertion into a transmission rack at a facility of the service provider.
The modulated optical signal is then transported on a first optical link 22A generally for a distance of about 60-80 km where the signal has become so deteriorated or degraded in quality (depending, in part, the type of fiber medium in use) that regeneration or REGEN of the optically propagating signal was necessary, e.g., the signal must be converted back into the electrical domain, digitally re-amplified, reshaped and/or retimed (2R or 3R) and then reconverted back into the optical domain (OEO) for transmission on the next optical link 22B. Such OEO REGENs 20, as shown along optical span 22 of FIG. 1, included the deployment of an optical modules 23 or 25 to receive (or transmit) the optical signal from terminals 12 or 16. These modules employ discrete, optically coupled optical active and passive components inside packages 23 and 25 and electronic linecards 21 and 24 for conversion of the optical information signal into the electrical domain, performing a 2R or 3R function, and then reconverting or regenerating the electrical domain signal back into the optical domain for continued propagation on the optical span 22. REGENS 20 may also have the capability of signal switching or routing to other networks or to local networks as indicated at 26. This OEO REGEN 20 was required between fiber spans 22B, 22C, 22D and 22E, which spans may typically be about 60 to 80 km in length, until the information signal reached its destination at terminal 16 (or terminal 14 depending on the direction of signal propagation) where the signal was converted into the electrical domain via optical module 16, comprising an optical detection element or component, such as p-i-n photodiode (PIN) or an avalanche photodiode (APD), and the signal was then further processed, such as amplified or reshaped, for example. As indicated, such an optical transport network 10 was generally bidirectional in that module 12 also included components of module 16 and visa versa in order that optical information signals can be sent and received in either direction along the fiber link 22. Such networks were capable of transporting a single optical signal, λI and/or λJ, e.g., at 2.5 Gb/sec or less, in either direction. However, to implement the network for signal transport, it was necessary to transport those signals respectively on a separate fiber, although there were systems that did utilize 1300 nm in one direction and 1500 nm in the other direction on the same fiber. Thus, there were bundles of fibers each supporting transport of a single modulated optical wavelength signal.
It is interesting to note that the concept of an OEO REGEN, such as illustrated at 20 in FIG. 1, goes back well before their first viable commercial utilization. Examples of types of OEO REGENs can be found in U.S. Pat. Nos. 4,090,067 (issued 1978), 4,688,260 (issued 1987) and 4,948,218 (issued 1990).
FIGS. 3A-3E explains pictorially the 3R function such as provided in OEO REGEN 20. First, a shown in FIG. 3A, clean digital pulses comprising an optical channel signal 11 is transmitted or launched on an optical link. Due to optical nonlinearities and impairments incurred in propagation of the channel signal over the link, which are explained in further detail below, the signal becomes so distorted and degraded, as seen at 13 in FIG. 3B, that OEO REGEN is necessary to restore the integrity of the signal; otherwise, the signal cannot be accurately read or deciphered at the optical receiver. The first step in the 3R process is re-amplification of the distorted signal 11 as shown at 15 in FIG. 3C. The next step, as shown in FIG. 3D, is to reshape the signal as a digital optical channel signal 17. Note, however, that signal pulse is still impaired as indicated at 19 where the pulse timing reference is incorrect. Thus, in the last step of the 3R process, the signal is retimed, as shown in FIG. 3E as regenerated replica of digital electrical signal 11, which is subsequently converted into an optical signal of like form via an optical modulator in an optical transmitter module.
The capacity of optical transport networks was increased by deploying time division multiplexing (TDM) where lower bit rate information signals are converted into higher bit rate signals for transport as a single optical wavelength. The lower bit rate optical signal is combined with other lower bit rate signals into a higher bit rate signal and transported all as a single wavelength over a single fiber to a receiving terminal or node where the low bit rate signals are separated out of the higher bit rate signal at the receiving terminal.
The capacity of optical signal transport was further materially increased with the advent of wavelength division multiplexing (WDM) commercialized in about 1994 where multiple optical information signals of different wavelengths were optically combined for transport as a multiplexed optical signal on a single fiber. This was a large leap forward because now multiple optical signals could be placed concurrently on a single optical fiber span or link.
Even though WDM brought a large enhancement to digital signal transport, the significantly high cost of OEO REGENS prevented immediate deployment due to the need to rejuvenate the multiplexed signal gain over short optical fiber spans, such as 40 to 60 km. This meant that the multiplexed channel signal had to be demultiplexed into the individual different-wavelength information signals which were converted into the electrical domain, possibly both reshaped and retimed and then regenerated back into the optical domain (OEO). Then, in about 1995, the commercial deployment of erbium doped fiber amplifiers (EDFAs) along the optical transport path became a reality. With advent of EDFAs, optically multiplexed signals could be simultaneously amplified in the optical domain (OO) and optical signal propagation distances were extended with the deployment of EDFAs along the span and between REGEN sites, i.e., the number of required REGEN sites along a given optical span and link could be reduced by providing OO conversion through the deployment of EDFAs in lieu of OEO conversion. Thus, the deployment of EDFAs eliminated the need for closer optical link spacing deploying costly OEO REGENs through the employment of intervening EDFAs 46, which is illustrated in FIG. 2. The advent of EDFAs enabled a new set of optical network economics due to the ability to replace multiple OEO REGENs at a single site with a single EDFA. However, the employment of EDFAs is not completely inexpensive because these fiber amplifiers must be of highest performance with operating characteristics such as good gain flattening characteristics and low noise figure over the operational bandwidth of the multiplexed optical signals. Also, these types of optical amplifiers limit the spectral extent of the optical signals that can be successfully amplified because of their limited gain bandwidth. The longer the network or system reach via optical amplifiers, the higher the cost of the network or system.
As shown in FIG. 2, in the EDFA implemented, bidirectional optical transport network 30, the point of signal generation may be, for example, from either optical terminal or node 32 or 36 having respective linecards 34 and 38. Also, along the transport path, OEO REGENs 40 are provided for 3R signal regeneration. At terminals 32 and 36, plural optical channel signals are generated from electrical pulse information signals and the multiple optical signals are multiplexed and launched onto an optical fiber span, such as span 41A or 41C. During signal propagation, the multiplexed signals are amplified by a high performance EDFAs 46 located along the optical span 41A, 41B or 41C. Initially, as an example, there may be about three to five such EDFA sites per span but, over time, the number in many cases could be increased to ten EDFA sites per span. When the multiplexed channel signals reached a REGEN 40, the multiplexed optical signals undergo demultiplexing followed by OE conversion, regeneration, reshaping and retiming (3R) via linecards 42 and 44 in the electrical domain and, then, EO conversion of the 3R generated signals performed in the transceiver modules 43 and 45 is accomplished for launching the optically rejuvenated or regenerated multiplexed channel signals on the next optical fiber span, such as span 41B. Also, these REGEN sites 40 include switching and routing capabilities at 46 so that channel signal can be switched, or routed or received from other nodes or sites for transmission from or into network 30. Multiplexed channel signals arriving at a terminal 32 or 36 undergo demultiplexing at 32A and 36A and OE conversion for electrical signal processing. Thus, each of the optical modules 32A and 36B has both OE and EO components so that optical signals can be transported in either direction of the optical span 41A, 41B and 41C.
The principal achievement of EDFA deployment, as indicated above, was the insertion of bidirectional EDFAs 46 along the optical link thereby eliminating the need for multiple REGENs and extending the length of the optical transmission up to about 100 km and longer before OEO REGEN becomes a real necessity. Thus, the optical spans 41A, 41B and 41C between OEO REGENs 40 could be extended by including optical amplification (OO) in either direction via EDFAs 46. This greatly enhanced the optical signal capacity.
In all of the different types of optical transport networks 30 having different channel capacity and/or different baud rate, the optical module and OEO REGEN architecture was principally the same, which is exemplified in FIG. 4 illustrating the types of discrete optical components that are found in typical optical modules terminals 32 and 36 and OEO REGENs 40. In some cases, the modulated sources comprise separate laser sources and modulator sources and in other cases, the modulated sources are integrated electro-optical modulator/lasers (EMLs). The case we show here is former case. Within each Tx module 50 and Rx module 60, a plurality of discrete optical components is utilized. While FIG. 4 shows Tx module 50 and Rx module 60 in separate packages, it is known to utilize both of these optical modules in the same package to form an optical transceiver or transponder. Each channel signal is EO generated or EO translated using individual optical components comprising a discrete channel laser and discrete channel modulator. However, the deployment of arrays of optical components is possible, e.g., a DFB laser bar of N laser emitters. But it is difficult to achieve multiple wavelength arrays of DFB lasers in a commercially practical manner because the wavelengths of the individual laser emitters must be of different wavelengths approximated or optimized to a standardized wavelength grid, such as the ITU grid, and their individual wavelengths stabilized against changing environmental and other operational conditions. For these reasons, the most reliable architecture to date is to deploy separate discrete tunable DFB laser components. These discrete optical components must be optically coupled to one another, which is highly expensive since such alignments, as identified at 51 and 61 in FIG. 4, must, in many cases, be carried out by hand for each of the optical coupling points.
Tx module 50 includes, for example, a plurality of discretely packaged DFB lasers 52(1) . . . 52(N-1) and 52(N) which have a wavelength stabilization control to maintain their individual operational wavelengths optimized to a standardized wavelength grid. Each of the N DFB lasers 52 is optically coupled via a fiber to the input of a discretely packaged modulator 54(1) . . . 54(N-1) and 54(N), which is generally a Mach-Zehnder modulator (MZM). The outputs of each of the N MZMs 54 are then optically coupled via a fiber to the input of MUX 56. Currently, MUX 56 is, generally in most cases, are simple fiber combiners, thin film-based filters, fiber Bragg gratings, silica-based AWGs and silicon-based AWGs, although there are possibilities, such as optical circulators and polarization-based multiplexers. The output of MUX 56 is then optically coupled usually to some gain medium functioning as a preamplifier 58 to boost the multiplexed signals to higher amplitude due to insertion loss experienced in optical components 54 and 56 as well as optical loss experienced at optical coupling points 51 between these optical components (including optical coupling to DFB lasers 52). The preamplifier is generally a high performance EDFA. In general, today, the channel signals are transmitted with 50 GHz channel spacing.
In the configuration of FIG. 4, the multiplexed optical signal channels are launched on optical fiber link 57 which may include a plurality of EDFAs 59 spatially disposed along its length to amplify or provide gain to the multiplexed optical signals, as previously indicated in FIG. 2.
Rx module 60 generally includes a preamplifier 62, which is generally an EDFA. After pre-amplification, the received multiplexed signals are optically coupled via a fiber to the input of MUX 64, which, as mentioned previously relative to Tx module 50, is currently a silicon-based AWG. DEMUX 64 may also include additional filters to convert the 50 GHz channel spacing of the signals into 100 GHz channel spacing. The demultiplexed signals are then optically coupled via optical fibers to respective photodetectors 66(1) . . . 66(N-1) and 66(N) for conversion into the electrical domain. The photodetectors generally utilized are of the APD or PIN type. The linecard (not shown) then processes these converted signals. Again, there are a number of optical coupling points 61 where fiber connections must be made to optically couple plural optical components to one another.
WDM optical transport networks of the type shown in FIG. 2 began to evolve by first increasing the capacity, i.e., the number of signal channel wavelengths of the networks, which reduced the number of parallel networks, and, thus, optical regenerators required to support a traffic demand at a particular site.
Reference is made to FIG. 5 which exemplifies the progression of price or cost reduction in optical transport pricing with the advent WDM transport networks or systems with corresponding increase in channel capacity and drop in cost per channel between about the years of 1994 and 2002. As shown in FIG. 5, the first transport networks carrying a single wavelength in a single direction with 3R and OEO REGEN, illustrated in FIG. 1, were of high cost in 1994 as indicated at point 70 in FIG. 5. While the concepts and prototypes of WDM networks began to become a reality in 1994, they were not yet commercially practical until the advent of the optical fiber amplifier or EDFAs which significantly extended the network reach as well as capacity. The WDM transport networks were around 1996 with the advent of 2.5 Gb WDM systems at point 71. Between roughly 1995 and 1998 (i.e., between about points 71 and 72 of FIG. 5) and forward, 2.5 Gb WDM systems went from 8 channels to 16 channels to 80 channels. As the WDM systems grew in capacity, the cost of these systems and cost per channel steadily decreased as shown in FIG. 5. Between points 72 and 73 in FIG. 5 or roughly from 1997 and forward, 10 Gb systems came into operation further increasing capacity through increased data rates and correspondingly reducing the cost of the systems and the cost per channel. In this period of time, the 10 Gb systems went from 8 channels to 32 channels to 80 channels and eventually to 160 channels extending toward point 74 in FIG. 5. At about point 73 or around 2001, the concentration by vendors and service provider customers has been toward extending system reach through the deployment of more and more EDFAs along the optical spans as well as the deployment of counter-propagating Raman amplification, rendering the distance between OEO REGEN sites or nodes further and further apart, further reducing the cost per channel, but not at a rate of cost per channel as initially experienced in the past as seen between points 70 73 as compared to point 74 in FIG. 5.
While the capacity increases were effective in improving the cost-effectiveness of optical transport networks, the networks were able to improve their cost-effectiveness by a larger factor by increasing the network reach. The reach of the network was defined by the maximum distance that the optical fiber amplifier could continue to support an appropriate OSNR for the channel signals. Initially, the distances that service providers were attempting to reach between traffic locations (“routes”) were typically much longer than the networks could support. Early networks were capable of reaching distances typically on the order of about 300-500 km. Routes today commonly reach distances exceeding 1000 km. Thus, multiple networks have to be concatenated together to reach other appropriate networks and terminations. Network costs were dominated by the electronic interfaces required at terminals 32, 36 and OEO REGEN sites 40 along long-haul routes. Thus, carrier providers were motivated to increase the reach of their networks to minimize the need for terminals 32, 36 and OEO REGENs 40. Over time, carrier providers were able to extend the reach of their optical transport networks to thousands of km.
The net effect over time on a typical route of 1000 km or any other extended system reach of a route even in excess of 1000 km, the increase of channel counts and distances has led to the following approximated economics savings relative to a normalized price:
TABLE 1ApproximatedYearEconomic SavingsOptical Network1994XSONET OEO REGEN19960.340X1ST Generation WDM19980.170X2ND Generation WDM20000.057X3RD Generation WDM20010.046X4TH Generation WDM
Said another way, the cost of optical signal transport has fallen by over a factor, for example, of over approximately 20 times in about the last seven years. However, along with these gains in deploying WDM systems in optical transport networks has come increasing complexity, together with additional costs, through the need for additional network components, higher performance and new constraints imposed by new and improved network devices, such as those for correcting nonlinearities and analog impairments. Continued increase in performance comes disproportionately with cost, which results in diminishing price/performance gains.
Thus, since network capacity has been largely addressed through these WDM transport systems carrying increasing larger number of signal channels, service providers now look to achieve economic gains by extending the reach of systems in their optical transport networks. This goal is based upon the premise that channel signal regeneration is costly and expensive. In order to achieve this goal, therefore, one solution is to eliminate OEO REGENS within the extended network reach. As result, there has been recent resurgence to provide an all optical transport network with OOO sites and OOO REGEN sites to eliminate costly OEO REGEN. This is because OEO REGENS are so much more expensive predominately because of the deployment of discrete optical components and their associated optical coupling point costs as well as 3R signal revamping in the electrical domain and subsequent signal regeneration in the optical domain via optical channel signal demultiplexing and multiplexing to perform full signal conditions. In this context, many companies and research facilities are working toward the development of an all-optical transport platform that is entirely in the optical domain, thereby eliminating any requirement for OEO conversion by its total elimination. A few examples of this work in this area are exemplified in U.S. Pat. No. 6,169,616 (OOO A/D), U.S. Pat. No. 6,335,819 (OOO REGEN), U.S. published patent application 2002/0015201 (OO spectrum attenuators) and U.S. published patent application 2002/0030867 (OO switching).
While the strides made toward an all-optical transport platform have been impressive, the ultimate achievement of a commercially viable all-optical transport platform is still many years away. Also, such OOO systems contemplate the continued deployment of many optical fiber amplifiers (EDFAs) between optical regen sites. This architecture has the continued analog drawback of wavelength dependent gain and noise as well as a variety of other non-linear impairments induced during the interrupted propagation down the fiber, causing unequal channel performance and, therefore, requires a multitude of added optical or opto-electronic devices and components to correct for their analog disparities such as gain flattening and dispersion compensation (DC). As an example, the above mentioned patent publication 2002/0015201 recognizes this problem and provides for signal attenuation for each channel to achieve equalized gain across the signal channel spectrum and likely lower OSNR but at the sacrifice of adding additional costly components to the system and still not eliminating accumulated channel signal imparities enhanced by cascaded analog optical fiber amplification in the transmission path.
Cost analysis of OEO REGEN, as known by those skilled in this area of technology, will reveal that these current packages utilized in the optical modules 50 and 60 a shown in FIG. 4 are the single highest manufacturing module cost in fabricating an optical transport network, with a leading cost factor in these modules being number of required optical fiber or other such coupling points 51 and 61. Multiplied by the number of such points 51 and 61, this is a major cost in the manufacture of optical modules for use in optical transport networks such as the network illustrated at 30 in FIG. 2.
What is needed as an alternative to the costly terminals an all optical network is an architectural approach to significantly reduce these high costs in optical module fabrication to provide a more palatable optical Tx or Rx module for terminals OEO REGEN for highly cost-conscience telecommunication service providers. Such new terminals and OEO REGEN sites need to be highly cost effective and a highly price competitive with both exiting OEO REGEN sites as well as OO sites such as the current deployment of EDFA sites along the optical fiber span.
As a review again in connection with the optical transport network 30 in FIG. 2, 1R signal treatment has been accomplished in the optical domain utilizing optical fiber amplifiers such as EDFAs and Raman amplifiers. To correct for signal attenuation, prior to any required OEO REGEN, EDFAs and Raman amplifiers have been deployed along the optical transport route toward extending the length or distance of digital signal propagation without significant signal impairment thereby significantly extending the distance required before providing any signal REGEN. To correct for optical signal impairments, such as GVD, signal impairment correction in the optical domain has been achieved through the employment of dispersion shifted fibers (DSFs) and optical dispersion compensating elements (DCE) toward further extending the length or distance of signal propagation without significant signal impairment thereby significantly extending the distance required before providing any signal OEO REGEN. In the case of the FIG. 2 architecture, in addition to the utilization of plural optical amplifiers (EDFAs or Raman amplifiers or other analog type amplification) EDFAs along the signal transport route to enhance signal gain, spectrum gain flatteners, VOAs and DCFs are employed to correct for signal nonlinearities and irregularities. However, in all these cases, complete signal REGEN in the true sense is not accomplished but rather signal amplification and some signal reshaping (2R) so that many analog-type signal impairments are still present and remain without such complete signal 3R OEO REGEN. What is needed is a cost effective way to provide OEO REGEN that replaces EDFAs and also provides for upgrade of optical amplifier sites to perform other functional capabilities such as signal routing or switching, multiplexing add/drop and grooming. By providing cost effective terminals and OEO REGEN sites in a new network, displacing existing OEO REGEN and optical fiber amplifier (OO) sites, the channel signal integrity is greatly enhanced, such as reduced BER reducing the requirement for more higher cost and more sophisticated FEC components and eliminating the need for optical signal conditioning components such as VOAs, gain flattening filters, DCF, etc., to name a few, and time extend for the replacement of existing transport fiber with newer higher performance optical fiber.
Thus, said another and possibly a more simplified way, digital optical network relative to this disclosure means complete digital signal REGEN in a highly cost effective and competitive manner to replace both existing REGEN sites and OO (EDFA) analog sites in both current and future optical transport networks.